Filter upstream_flank NOT FOUND sometimes happens sometimes doesn't

[Ray]

Dear Sir/Madam:

When I was following the "Given a set of EntrezGene identifiers, retrieve 100bp upstream promoter sequences" in The BiomaRt  user guide,

seq=getSequence(id=c('7407','7264'), type='entrezgene', seqType ='coding_gene_flank', upstream = 200, mart = ensembl)

it raises the error, 

Error in getBM(c(seqType, type), filters = c(type, "upstream_flank"),  : 
  Query ERROR: caught BioMart::Exception::Usage: Filter upstream_flank NOT FOUND

But sometimes it just works and return the 100bp upstream promoter sequences.

Don't know why does this error come out casually.

Also, when I retrieve the 100bp upstream promoter sequences of entrez gene 7407 and 7264,  it returns more than 1 sequences.


                                       coding_gene_flank entrezgene
533    CCTCTGAGAGCCAGGTCCTCGGCTCCTGACAGCCTGCCTGGTCAGA...       7407
587    NNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNN...       7407
12137  TGTAGGCTGAGAGCTCCCAGGGGAACAGGGCTGCGACGAGGGAGTG...       7264
12271  CTCCCCTAAGGGCTGGCCCTGGGGCCCTGCCGGGGGAGCCGGAGCA...       7264


why dose it return more than 1 sequence for each entrez id? and why is one of the sequences of 7407 all 'N's?

Appreciate that if you can answer my question!
Thanks in advance!

Best regards,
Ray